The long term objective of this work is to provide the capability of performing x-ray transmission CT imaging nearly simultaneously with PET imaging. This will provide improved data for attenuation correction and will greatly reduce what has been a primary source of error in PET reconstructions. If the quality of transmission CT data is high, reconstructed images will be useful as guides for precisely determining anatomic sites of radiotracer uptake. Additionally, reconstructed images may be useful as guides for precisely determining anatomic sites of radiotracer uptake. These anatomic maps would be an extremely useful adjunct to tumor images acquired using PET in conjunction with 18FDG. Emphasis will be on developing a low-cost approach; specifically, cone-beam scanning using a low-power x-ray source in conjunction with an amorphous silicon imaging detector will be investigated. Amorphous silicon detector arrays are the largest pixellated imaging devices presently built. They are based on the same technology as that used to create active-matrix flat-panel displays. The specific aims are 1) evaluate and optimize performance of amorphous silicon imaging arrays for x-ray transmission CT; 2) implement appropriate cone-beam scanning and reconstruction techniques; 3) develop methods for estimating PET attenuation correction factors from x-ray CT data with particular emphasis on minimizing necessary x-ray dose. The goal for the present project period is to demonstrate the feasibility of this concept and to develop and validate techniques that will be used to design a prototype of a combined x-ray CT and PET system in a subsequent project period. Results of this investigation will have application to oncology and may have application for both SPECT and radiotherapy treatment.